The present invention relates to a method and a device for controlling at least one physical parameter of an optical signal.
Such a method and such a device may be used, though not exclusively, to stabilise or at least to control the polarisation state—or any other physical parameter such as a spatial propagation mode or a wavelength—of an optical signal.
A plurality of fundamental physical parameters of an optical signal, such as the polarisation state, the spatial modes and the wavelengths, are capable of varying during the propagation thereof. This is because, when the optical signal is propagated, for example, over several kilometers of optical fibre, the birefringence—even slight birefringence—thereof is sufficient to randomly modify the polarisation state of the signal, which makes it impossible to predict it.
In order to stabilise or at least to control the polarisation state of an optical signal, it is already known to use a dissipation system, such as a Glan polariser, which allows an optical signal having constant polarisation to be obtained at the output. However, this stabilisation is brought about to the detriment of the level of intensity of the signal in that the level of intensity then depends on the polarisation state and therefore fluctuates at the same time as the polarisation fluctuates.
Furthermore, it is also known to use an electronic retrocontrol system which comprises a first control element, an optical control element, which allows any polarisation state to be converted into another polarisation state without any loss. A second element, an electronic element, allows the resultant polarisation state to be measured and analysed in order subsequently to transmit to the first optical element instructions in order to stabilise the final polarisation state at a predetermined state.
Owing to the high response time of the second electronic element, however, the use of this type of electronic prior retrocontrol system is limited to controlling optical signals whose polarisation varies at a low rate.
In order to stabilise, without any random loss of light intensity, an optical signal whose polarisation state varies at any rate, the document of patent FR 2 950 164 has proposed a completely optical control system (therefore, without any electronic components) for the polarisation state of an optical signal. In this document, the optical signal is propagated in an optical waveguide which has Kerr type non-linearity and a weak dispersion of the polarisation modes. Furthermore, there is transmitted into this optical waveguide, on the one hand, via its input, the initial optical signal and, on the other hand, via its output, an optical control signal which is counter-propagated relative to the initial optical signal whose polarisation state is constant and whose spectrum is adapted to minimise the Brillouin scattering in the optical waveguide. In this manner, the non-linear interaction which is produced in the guide between the initial optical signal and the counter-propagating optical signal whose polarisation state is constant allows the constant polarisation state of the output optical signal to be imposed on the initial optical signal when the optical signal is discharged from the optical waveguide via its output.
However, this prior technique has the disadvantage of requiring a source which is capable of generating the counter-propagating optical signal which involves, at the same time, a complex configuration and a high cost. Furthermore, it has been found that, with this prior technique using a counter-propagating optical signal, the control of the polarisation state of the optical signal, after it is introduced into the optical waveguide, is sensitive to the local stresses to which the optical waveguide is subjected, which impairs the stabilisation of the polarisation state.
Furthermore, the document US 2007/103684 A1 describes a completely optical method for controlling a physical parameter of an initial optical signal, in accordance with which there is transmitted into an optical waveguide, via its input, the initial optical signal and, via its output, an optical control signal which is counter-propagated relative to the initial optical signal in order to stabilise or at least to control the physical parameter of the initial optical signal when it is discharged from the optical waveguide via its output, this method being remarkable in that the initial optical signal which has passed through the optical waveguide from the input as far as the output thereof is at least partially returned into the optical waveguide in order to form at least partially the optical control signal.
In this manner, the counter-propagating optical signal is obtained directly from the initial optical signal which has passed through the optical waveguide at least once, which prevents introduction of an independent counter-propagating optical signal by means of a source which is specifically provided for that purpose. A simple optical return element may be sufficient to take a portion of the initial optical signal which has passed through the optical waveguide and to return it into the guide, the remainder of the initial optical signal being recovered at the output of the control device which the guide and the optical return element form.
In this instance, it has been found that, with regard to the polarisation state of an optical signal, the fact of using a portion of the initial optical signal which has passed through the optical waveguide as a counter-propagating optical signal produces a stabilising effect for the polarisation state similar to the effect produced by a counter-propagating signal having a constant polarisation state which is generated by an independent light source. Consequently, in the absence of a constant polarisation state which is intended to be imposed on the initial optical signal, the signal is progressively stabilised towards a particular, specific stable state of the optical waveguide and the initial polarisation state of the initial optical signal. More specifically, it has been found that, in the region of the output of the optical waveguide (located downstream in relation to the propagation direction of the initial optical signal), the optical signal rapidly converges towards one polarisation state from two possible states. In an isotropic optical fibre, those two states correspond to the left-hand and right-hand circular polarisations whilst, in a non-isotropic optical fibre having weak residual birefringence, they remain orthogonal but the precise values thereof depend on multiple parameters of the fibre, in particular the winding, the linear and circular residual birefringences thereof, etc. The optical signal converges towards the state of the two polarisation states that is nearest its initial polarisation state.
Furthermore, the stabilisation of the desired physical parameter—for example, the polarisation state—is not very sensitive to the local stresses to which the waveguide is subjected, which allows even more reliable stabilisation.
According to the method of the document US 2007/103684 A1, the initial optical signal which has passed through the optical waveguide from the input as far as the output thereof is partially returned into the optical waveguide by means of a reflective element which may be, for example, a mirror which is arranged at the output of the optical waveguide, or a reflective treatment which is applied to the output face of the optical waveguide. Therefore, the light intensity of the returned portion of the signal is less than that of the initial optical signal, which does not allow the linear interactions between those signals to be optimised. For example, the transmission coefficient of the mirror is in the order of 5%, which allows substantially equal intensities (95%) to be maintained between the counter-propagating signals in the optical waveguide to the detriment of the light intensity of the stabilised optical signal which has passed through the mirror (5%).